Thermocouple probe

ABSTRACT

Sheaths of thermally unstable material are applied to the portions of the thermocouple wires adjacent the thermocouple junction. The sheathed wires are embedded in encapsulating material and then exposed to a temperature sufficient to degrade the sheath material in a manner that reduces its volume. Subsequent thermal expansion and contraction of the conductors takes place without mechanical stressing in the resulting open spaces surrounding each conductor.

United States Patent 1 1 Sebestyen 1 1 Jan. 30, 1973 541 THERMOCOUPLEPROBE 2,609,406 9/1952 Barsy ..136/233 Inventor: Thomas M. Sebestyen AnnArbor, 3,159,032 12/1964 Rademacher et al ..l36/23l Mich- OTHERPUBLICATIONS Asslgnee: Motor Company Dearbom, Continental Sensing, lnc.,Con-O-Twist Cable Test Mw Data, 1966, pp 1-7 [22] Filed: Nov. 12, 1970Primary Examiner-Carl D. Quarforth [21] Appl 88774 AssistantExaminer-Harvey E. Behrend Attorney-John R. Faulkner and Glenn S.Arendsen [52] US. Cl ..136/233, 136/201 [51] int. Cl. ..H01v 1/02 [57]ABSTRACT 4 [58] Flew of Search 136/200 201 230 23 Shcaths of thermallyunstable material are applied to [56] References Cited the portions oithe thermocouple wires adjacent the thermocouple unction. The sheathedwires are em- UNITED STATES PATENTS bedded in encapsulating material andthen exposed to 3,121,038 2/1964 Perotte ..136/23() X i P Sufficient todegrade the Sheath materi' 3,329,766 7H9 Cole I I I I I l I l36/234X alin a manner that reduces its volume. Subsequent 2,509,325 g o Keyser I136/233 X thermal expansion and contraction of the conductors 1,827,25210/1931 Mollard ..136/233 X takes place without mechanical stressing inthe result- 3,057,941 10/1962 Schwartzwalder ..136/233 ing open spacessurrounding each conductor. 3,232,794 2/1966 Korton ..l36/233 2,517,0538/1950 Thompson .136/233 X 9 Claims, 4 Drawing Figures SHE U 10F 2INVENTOR 750/220: W7. fzes/e/r ATTORNEYS F'lG.l

PATENTEUJAm ms PATENTEDJAHSO I915 3.713899 SHEET 20F 2 INVENTOR Tia/220si2 4215x272 ATTORNEYS THERMOCOUILE PROBE SUMMARY OF THE INVENTIONControl systems for gas turbine engines require several temperaturesensing devices that must be capableof satisfactory operation over anextended time period. Such operation necessarily involves numerouscycles in which the temperature sensing device is heated to itsoperating temperature and subsequently cooled to ambient. The operatingtemperatures can exceed 2,000F. and the thermally generated stresses inthe thermocouple wires of such devices can result in early failure.

This invention provides a thermocouple probe capable of operating forextended time periods in an environment involving rapid temperaturefluctuations. In the probe, a pair of electrical conductors ofdissimilar metals are joined together at a junction located within asuitable metal housing. Each of the conductors has a thermally unstablesheath of significant thickness on its length adjacent to but notincluding the junction. A thermally conducting encapsulating materialfills the housing to embed the junction and thethermally unstablesheaths. After the encapsulating material is cured, the probe is exposedto a temperature at which the sheath material deteriorates or ismodified in a manner that reduces its solid volume. Small channels thusare formed in the encapsulating material around each elec tricalconductor and these channels permit thermal expansion and contraction ofthe conductors without imposing undue mechanicalstresses.

Temperature response is improved by reducing mechanically the diameterof the probe tip after the housing has been filled with encapsulatingmaterial but before the sheath volume has been reduced. The conductorscan be twisted loosely prior to being embedded in the encapsulatingmaterial to increase the dimensional flexibility of the conductors.

Useful sheath materials include those that are combustible at thepredetermined temperature or those that shrink upon exposure to thepredetermined temperature. Suitable materials of the former type areethyl cellulose, cellulose acetate, phenolic resins, polyvinyl acetate,polyamides, polyethylene, polypropylene, polystyrene, etc. Typical ofmaterials that merely reduce in volume are the expanded or foamedversions of these polymeric materials as well as some types of polyvinylchloride, polytetrafluoroethylene, silicone polymers, and others. Afterany gases produced by the sheath volume reductions have been dissipatedout of the probe, a thin seal of additional encapsulating material canbe applied to the top of the probe to avoid atmospheric deterioration orcontamination of the electrical conductors. In applications using sheathmaterials that do not produce significant amounts of gases during volumereductions, a composite sheath that inherently provides a seal can beused. The composite sheath can be a lower portion of thermally unstablesheath with an upper portion of thermally stable material. The upperportion of thermally stable material extends for a short distance intothe probe and is surrounded by encapsulating material that forms a tightseal therewith. In an alternative construction, the entire electricalconductor can be surrounded initially with a thermally stable sheathmaterial that has an outer coating of the thermally unstable materialfor the portion of its length adjacent the thermocouple junction.Encapsulating material forms a tight seal with the exposed thermallystable material and the volume of the outer coating of thermally unstable material is reduced as described above to provide the stressrelieving channels. Typical thermally stable materials include the hightemperature polyamides, polyimides, epoxy resins, and polyfluorocarbonsor inorganic materials such as asbestos and electrically insulatingceramics.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectioned elevation of athermocouple probe of this invention showing the relationship of theconductors and thermally unstable sheaths to the encapsulating compoundprior to volume reduction.

FIG. 2 shows the probe of FIG. 1 after the volume reduction has beeneffected and a thin layer of additional encapsulating material has beenapplied to seal the top of the probe.

FIG. 3 shows a probe having a composite sheath in which the sheathadjacent the junction is thermally unstable material and the sheathremoved from the junction is thermally stable material.

FIG. 4 shows a construction in which sheaths of thermally stablematerial surround the entire lengths of .the electrical conductors andeach sheath has a coating of thermally unstable material on its portionadjacent the junction; a I

DETAILED DESCRIPTION Referring to FIG. 1, the probe comprises anelongated cylindrical housing 10 that is closed at its lower end. Theends of a suitable pair of electrical conductors l2 and 14 are weldedtogether to form a thermocouple junction '16 that is positioned near thelower end of housing 10. Each conductor has a sheath 18 of thermallyunstable material surrounding its length adjacent to but not includingjunction 16.

To provide greater flexibility, conductors 12 and 14 are twisted looselyfor a short distance adjacent junction 16. One or two loose twists aresufficient for most purposes and many applications can be satisfied withonly partial twists. An encapsulating material 20 is packed into housing10 to fill completely at least the lower portion of the housing. Forprobes designed to operate at moderate temperatures, a polymericencapsulating material such as urea-formaldehyde,polyamide, polyimide,or other suitable polymers can be used. The encapsulating material caninclude small amounts of suitable thermally conducting fillers ifdesired. Encapsulating materials suitable for use in probes designed forhigh temperature applications can include a composite of inorganicfibers bounded together with an inorganic binder such as collodialsilica.

After the encapsulating material hasbeen packed tightly into the housingand has been cured, the tip portion indicated by numeral 22 of theassembly is swedged to a smaller diameter as shown in FIG. 1. The probethen is exposed to a temperature at which the volume of the sheathmaterials 18 is reduced. These volume reductions leave narrow channels24 around the electrical conductors as shown in FIG. 2. A thin layer 25of additional encapsulating material then is applied to the top of theprobe to seal the exit portions of the conductors. In the resultingassembly, the conductors are free to expand and contract in channels 24and thus do not generate undue mechanical stresses during cyclicoperation.

Turning to the construction shown in FIG. 3, the thermally unstablesheaths 18 extend for only a short distance from junction 16. Beyondthat distance a thermally stable sheath 26 surrounds each of theconductors. The thermally stable sheaths are embedded in at least aportion of the encapsulating material 20 which forms a tight sealtherewith. Upon exposure to a predetermined temperature, the thermallyunstable sheaths reduce in volume to leave channels around the lowerportions of the conductors but the thermally stable sheaths 26 areunaffected thereby and maintain a sealingrelationship with theencapsulating material. This arrangement can be used with thermallyunstable sheaths that produce gases during volume reductions byinitially filling the housing with encapsulating material to the loweredges of the thermally stable sheaths, effecting the volume reductions,and then applying the remainder of the encapsulating material.

In FIG. 4, thermally stable sheaths 26' surround the entire length ofconductors 12 and 14 except for junction 16. A thermally unstablecoating 28 is applied to the lower portions only of sheaths 26. Theconductors are embedded in encapsulating material as described above sothat encapsulating material contacts at least a portion of sheaths 26'.Upon exposure to the predetermined temperature, coatings 28 reduce involume to leave small channels. The thermally stable sheaths 26' areunaffected, however, and are free to flex in the channels.

In typical constructions suitable for gas turbine temperatures, probesinitially about 0.1 inch in diameter have tips reduced to approximately0.06 inch in diameter by a mechanical operation. One conductor is anickel-chromium alloy of the type available commercially as chromel andthe other is a nickel-aluminum alloy of the type available commerciallyas alumel. Each conductor has a diameter of about 15 mils and thermallyunstable polyethylene sheaths can be applied in thicknesses capable ofproviding volume reductions of about 3 mils. Volume reductions of about-30 percent of the diameter of the conductors generally are desirablefor applications involving large temperature variations.

The thermocouple junction can contact the probe housing in applicationswhere the probe housing is insulated electrically from the thermocouplecircuit. Encapsulating materials must be capable of withstanding highertemperatures than the thermally unstable sheath materials and must bechemically compatible with the sheath materials, but otherwise thevariety of useful materials is quite unrestricted. Polymers of the samebasic nature but with different temperature properties avoid problems ofchemical incompatability and thermal expansion differences; for example,the thermally unstable sheaths can be made of a low temperaturepolyamide while the encapsulating material and any thermally stablesheaths are made of high temperature polyamide.

Thus this invention providesa thermocouple probe having greatlyimprovedlife in a cyclical temperature environment. The probe isadaptable readily to the mass production techniques essential to broadcommercialization of gas turbine engines.

I claim:

l. A thermocouple probe comprising a housing,

a pair of electrical conductors of dissimilar metals joined together ata junction within said housing, each of said conductors having athermally unstable sheath of significant thickness on its length adjacent to but not including said junction, said thermally unstablesheaths reducing in volume when exposed to a temperature exceeding apredetermined minimum but less than the maximum operating temperature ofthe probe, and

encapsulating material filling said housing and embedding said junctionand the lower portions of said thermally unstable sheaths so thatexposing said lower portions of said sheaths to a temperature exceedingsaid predetermined minimum but less than the maximum operatingtemperature of the probe leaves channels around said conductors, saidchannels providing dimensional flexibility for said conductors.

2. The thermocouple probe of claim 1 in which the housing has an endportion of reduced size and the conductors'are twisted loosely for theentire length of said end portion.

3. The thermocouple probe of claim 2 in which the sheaths are made of apolymeric material that is thermally degradeable at said minimumtemperature.

4. The thermocouple probe of claim 3 in which each thermally unstablesheath is capable of providing a volume reduction of about l030 percentof the thickness of its electrical conductor.

5. The thermocouple probe of claim 4 in which each electrical conductorcomprises an upper sheath spaced longitudinally along its length fromsaid junction by said first mentioned sheath, said upper sheaths beingthermally stable at the operating temperature of the thermocouple, saidencapsulating material sealingly contacting portions of said uppersheaths to produce gas tight seals around the upper portions of saidelectrical conductors.

6. The thermocouple probe of claim 4 in which each electrical conductorhas a thermally stable inner sheath surrounded by said thermallyunstable sheath and said encapsulating material sealingly contactsportions of said thermally stable inner sheaths.

7. The thermocouple probe of claim 1 in which each thermally unstablesheath has a thickness providing a volume reduction of 10-30 percent ofits conductor when exposed to a temperature exceeding said predeterminedminimum.

8. The thermocouple probe of claim 1 in which each electrical conductorcomprises an upper sheath spaced longitudinally along its length fromsaid junction by said first mentioned sheath, said upper sheaths beingthermally stable at the operating temperature of the thermocouple, saidencapsulating material sealingly contacting portions of said uppersheaths to produce gas tight seals around the upper portions of saidelectrical conductors.

9. The thermocouple probe of claim 1 in which each electrical conductorhas a thermally stable inner sheath surrounded by said thermallyunstable sheath and said encapsulating material sealingly contactsportions of said thermally stable inner sheaths.

1. A thermocouple probe comprising a housing, a pair of electricalconductors of dissimilar metals joined together at a junction withinsaid housing, each of said conductors having a thermally unstable sheathof significant thickness on its length adjacent to but not includingsaid junction, said thermally unstable sheaths reducing in volume whenexposed to a temperature exceeding a predetermined minimum but less thanthe maximum operating temperature of the probe, and encapsulatingmaterial filling said housing and embedding said junction and the lowerportions of said thermally unstable sheaths so that exposing said lowerportions of said sheaths to a temperature exceeding said predeterminedminimum but less than the maximum operating temperature of the Probeleaves channels around said conductors, said channels providingdimensional flexibility for said conductors.
 2. The thermocouple probeof claim 1 in which the housing has an end portion of reduced size andthe conductors are twisted loosely for the entire length of said endportion.
 3. The thermocouple probe of claim 2 in which the sheaths aremade of a polymeric material that is thermally degradeable at saidminimum temperature.
 4. The thermocouple probe of claim 3 in which eachthermally unstable sheath is capable of providing a volume reduction ofabout 10-30 percent of the thickness of its electrical conductor.
 5. Thethermocouple probe of claim 4 in which each electrical conductorcomprises an upper sheath spaced longitudinally along its length fromsaid junction by said first mentioned sheath, said upper sheaths beingthermally stable at the operating temperature of the thermocouple, saidencapsulating material sealingly contacting portions of said uppersheaths to produce gas tight seals around the upper portions of saidelectrical conductors.
 6. The thermocouple probe of claim 4 in whicheach electrical conductor has a thermally stable inner sheath surroundedby said thermally unstable sheath and said encapsulating materialsealingly contacts portions of said thermally stable inner sheaths. 7.The thermocouple probe of claim 1 in which each thermally unstablesheath has a thickness providing a volume reduction of 10-30 percent ofits conductor when exposed to a temperature exceeding said predeterminedminimum.
 8. The thermocouple probe of claim 1 in which each electricalconductor comprises an upper sheath spaced longitudinally along itslength from said junction by said first mentioned sheath, said uppersheaths being thermally stable at the operating temperature of thethermocouple, said encapsulating material sealingly contacting portionsof said upper sheaths to produce gas tight seals around the upperportions of said electrical conductors.